The present invention refers to a ferrous coating applied by a plasma spraying operation to a substrate serving as a cylinder working surface of a combustion engine block. Moreover, the invention also refers to a method of applying a ferrous coating to a substrate serving as a cylinder working surface of a combustion engine block.
In the prior art, the traditional material for the working surfaces of the cylinders of combustion engine blocks that are made of aluminum or magnesium alloy is constituted by grey cast iron or cast iron blended with compacted graphite. Thereby, cylinder sleeves made of such cast iron are pressed or cast into these combustion engine blocks.
By providing such cylinder sleeves, however, on the one hand the size and the weight of the engine block is influenced in a negative sense. On the other hand, an inconvenient or adverse connection between the cylinder sleeves made of cast iron and the engine block made of a light metal alloy must be taken into account. Alternatively, also coatings applied by a galvanizing process have been used. However, the application of such coating is expensive and, moreover, such coatings may corrode under the influence of sulfuric acid and formic acid.
Furthermore, the application of a coating to bores in general by means of a plasma spraying operation is known in the art for a long time. Thereby, a variety of metallic materials can be applied to the substrate. Once the coating has been applied by means of the plasma spraying operation, the bores are further processed by diamond honing to reach their desired final diameter and provided with the desired topography. The ability of the coating to be processed and machined, respectively, and the tribologic properties are depending to a high degree on the microstructure and the physical properties of the particular coating.
It is an object of the present invention to improve the machining and processing, respectively, as well as the tribologic properties of ferrous coatings for the working surfaces of combustion engine cylinder blocks applied by a plasma spraying operation.
In order to meet this and other objects, the invention provides, in a first aspect, a ferrous coating applied by a plasma spraying operation to a substrate serving as a cylinder working surface of a combustion engine block, whereby the coating has a content of bound oxygen of between 1% and 4% by weight.
The invention is based on the surprising observation that a microstructure can be created by means of a specially controlled reaction of the powder used for the coating and oxygen during a plasma spraying operation, i.e. a microstructure comprising outstanding properties as far as machining and processing, respectively, as well as tribology are concerned. Particularly, the coefficient of friction and the tendency towards scuffing, i.e. the beginning of adhesive wear, are drastically decreased.
As previously mentioned, the coating of the invention, applied by plasma spraying, has a content of bound oxygen of between 1 and 4% by weight. As a substrate for applying such a coating, particularly suitable are:
the cylinder bores of combustion engine cylinder blocks made of an aluminum or a magnesium alloy or of cast iron;
the inner wall of sleeves made of cast iron and inserted into a combustion engine cylinder block made of an aluminum or a magnesium alloy.
In a preferred embodiment, the bound oxygen forms, together with the iron, FeO and Fe3O4 crystals in the coating. Thereby, it is preferred that the content of Fe2O3 amounts to less than 0.2% by weight. The amount of the formed oxides can be further controlled by mixing the air with nitrogen or oxygen. If the air is replaced by pure oxygen, the content of bound oxygen in the coating is reduced by a factor of about two.
In a second aspect, the invention also refers to a method of applying a ferrous coating to a substrate serving as a cylinder working surface of a combustion engine block. The method comprises the steps of providing a plasma spraying apparatus, providing a coating powder constituting the raw material of the coating to be applied, spraying the coating powder by means of the plasma spraying apparatus onto the cylinder working surface; and either
supplying air to the plasma spraying apparatus and spraying the air simultaneously with the coating powder onto the substrate in an amount of between 200 and 1000 normalized liters per minute; or
supplying an oxygen containing gas to the plasma spraying apparatus and spraying the oxygen containing gas simultaneously with the coating powder onto the substrate in an amount of between 40 and 200 normalized liters oxygen per minute; or
supplying oxygen to the plasma spraying apparatus and spraying the oxygen simultaneously with the coating powder onto the substrate in an amount of between 40 and 200 normalized liters per minute.
The expression xe2x80x9cnormalized liters per minutexe2x80x9d shall be understood as xe2x80x9cliters per minute at an ambient pressure of 1 bar (=105 Pa) and a temperature of 20xc2x0 C. Preferably, the velocity of the gas flow in the interior of the sleeve or cylinder bore amounts to between 7 and 12 m/s during the plasma spraying operation.
In a preferred embodiment, a gas atomized powder is plasma sprayed to the substrate, whereby the powder has the following composition:
C=0.4 to 1.5% by weight
Cr=0.2 to 2.5% by weight
Mn=0.02 to 3% by weight
P=0.01 to 0.1% by weight, if appropriate
S=0.01 to 0.2% by weight, if appropriate
Fe=difference to 100% by weight.
In another preferred embodiment, a gas atomized powder is plasma sprayed to the substrate, whereby the powder has the following composition:
C=0.1 to 0.8% by weight
Cr=11 to 18% by weight
Mn=0.1 to 1.5% by weight
Mo=0.1 to 5% by weight
S=0.01 to 0.2% by weight, if appropriate
P=0.01 to 0.1% by weight, if appropriate
Fe=difference to 100% by weight.
The amount of FeO and Fe3O4 in the coating can be influenced by the distribution of the size of the particles of the powder. Depending on the coating to be realized, the size of the particles of the powder can be in the region of between 5 to 25 xcexcm, in the region of between 10 to 40 xcexcm, or in the region of between 15 to 60 xcexcm. The size of the particles can be determined by means of an optical or an electronic microscope, particularly by means of a scanning microscope, or according to the laser diffraction method MICROTRAC.
Preferably, a coating powder is used that has been gas atomized by means of argon or nitrogen.
The best results can be obtained if a coating powder is used that is blended with a tribologic oxide ceramics. Preferably, the oxide ceramics consists of TiO2 or Al2O3TiO2 and/or Al2O3ZrO2 alloy systems. The portion of the oxide ceramics in the coating powder can amount to between 5 and 50% by weight.
It should be noted that the optimum particle size is selected according to the tribologic properties of the coating to be applied and according to the mechanical behavior of the substrate to which the coating has to be applied.